In recent years, plate heat exchangers have been gradually promoted in various industries due to their lightweight, small footprint, low investment, high heat exchange efficiency, flexible assembly, and ease of scale removal. However, because the flow cross-section of plate heat exchangers is relatively small, scaling can easily lead to blockages, reducing the heat exchange efficiency and affecting the safety of the equipment and normal use by users. Therefore, solving the cleaning of plate heat exchangers and preventing the formation of scale will become an important issue to ensure safe production and economic operation.
During the use of plate heat exchangers, improper operation of water treatment equipment and failure to meet water quality control standards can lead to unqualified softened water being injected into the system. This causes calcium, magnesium, and carbonate salts in the water to decompose into calcium carbonate and magnesium hydroxide precipitates upon heating, which adhere to the heated surfaces of the heat exchanger, forming hard scale. Due to the poor thermal conductivity of the scale, the heat exchange efficiency of the exchanger decreases, resulting in significant waste of thermal energy, thereby affecting the heat transfer effect.
Cleaning methods for scale in plate heat exchangers
1. Selection of cleaning agents
Currently, the cleaning agent used is acid cleaning, which includes organic acids and inorganic acids. The main organic acids are oxalic acid, formic acid, etc. The main inorganic acids are hydrochloric acid, nitric acid, etc. Based on the analysis of the scaling of the heat exchanger and the process, material, and composition of the scale, it is concluded that:
1) The flow area of the heat exchanger is small, and the internal structure is complex, making it difficult to discharge any precipitate that may form in the cleaning solution.
2) The material of the heat exchanger is nickel-titanium alloy, and using hydrochloric acid as the cleaning solution can easily cause strong corrosion to the plates, shortening the lifespan of the heat exchanger.
Through repeated experiments, it was found that formic acid is the most effective cleaning solution. Adding a buffer and surfactant to the formic acid cleaning solution improves the cleaning effect and reduces the corrosion of the cleaning solution on the plates. Chemical tests on scale samples have shown that formic acid can effectively remove scale. Through soaking tests with acid solutions, it was found that formic acid can effectively remove scale attached to the plates, and it has minimal corrosive effects on the heat exchanger plates.
2. Basic principles for removing scale
1) Dissolution: Acid solutions easily react with calcium, magnesium, and carbonate scale to form soluble compounds, causing the scale to dissolve.
2) Peeling: Acid solutions can dissolve oxides on the metal surface, breaking the bond with the scale, thus causing the scale attached to the metal oxides to peel off and fall away.
3) Gas lifting: After the acid solution reacts with calcium, magnesium, and carbonate scale, a large amount of carbon dioxide is produced. During the release of carbon dioxide gas, it exerts a certain lifting force on the difficult-to-dissolve or slowly dissolving scale layers, causing the scale to fall off the heated surfaces of the heat exchanger.
4) Loosening: For mixed scales containing silicates and sulfates, the calcium, magnesium, carbonates, and iron oxides dissolve in the acid solution, causing the remaining scale to become loose and easily washed away by the flowing acid solution.
3. Process requirements for cleaning scale
1) Acid cleaning temperature: Increasing the acid cleaning temperature is beneficial for improving the descaling effect. However, if the temperature is too high, it will exacerbate the corrosion of the heat exchanger plates by the acid solution. Through repeated experiments, it was found that the acid cleaning temperature should be controlled at 60°C.
2) Concentration of the acid cleaning solution: Based on repeated experiments, the acid cleaning solution should be prepared with a concentration of 81.0% formic acid, 17.0% water, 1.2% buffer, and 0.8% surfactant, resulting in excellent cleaning effects.
3) Acid cleaning method and time: The acid cleaning method should combine static soaking and dynamic circulation. The acid cleaning time should start with static soaking for 2 hours, followed by dynamic circulation for 3 to 4 hours. During the acid cleaning process, samples should be taken regularly to test the acid concentration. When the difference in concentration between two consecutive tests is less than 0.2%, the acid cleaning reaction can be considered complete.
4) Passivation treatment: After acid cleaning, most of the scale and metal oxides on the surface of the plate heat exchanger are dissolved and fall off, exposing new metal, which is highly susceptible to corrosion. Therefore, passivation treatment should be performed on the heat exchanger plates after acid cleaning.
4. Specific steps for cleaning scale
1) Rinsing: Before acid cleaning, the heat exchanger should be rinsed with an open system to ensure that there are no mud, scale, or other impurities inside, which can improve the effectiveness of acid cleaning and reduce the amount of acid consumed.
2) Pour the cleaning solution into the cleaning equipment, and then inject it into the heat exchanger.
3) Acid cleaning: The heat exchanger filled with acid solution should be soaked statically for 2 hours, followed by continuous dynamic circulation for 3 to 4 hours. During this time, alternate cleaning should be performed every 0.5 hours. After acid cleaning, if the pH of the acid solution is greater than 2, the acid solution can be reused; otherwise, it should be diluted and neutralized before being disposed of.
4) Alkaline cleaning: After acid cleaning, prepare a mixture of NaOH, Na2PO4, and softened water in a certain ratio, and use dynamic circulation to perform alkaline cleaning on the heat exchanger to achieve acid-base neutralization, preventing further corrosion of the heat exchanger plates.
5) Water washing: After alkaline cleaning, use clean softened water to repeatedly rinse the heat exchanger for 0.5 hours to thoroughly wash away any remaining residues.
6) Recording: During the cleaning process, strict records of the time for each step should be kept to check the cleaning effect. In summary, after cleaning, a pressure test should be conducted on the heat exchanger, and it can only be used after passing the test.
3. Measures to prevent scaling in plate heat exchangers
1) Strictly control water quality during operation; water in the system and softened water in the softening tank must undergo rigorous water quality testing and can only be injected into the pipeline network after passing.
2) When a new system is put into operation, the heat exchanger should be separated from the system for a period of circulation before being integrated into the system to avoid impurities in the pipeline network entering the heat exchanger.
3) In the entire system, in addition to the periodic cleaning of the decontaminator and filter, the cleanliness of the pipeline network should also be maintained to prevent the blockage of the heat exchanger.
In summary, strictly following the cleaning method of the plate heat exchanger is an important guarantee for the normal operation of production.
